The brown spots in the sand by Diarrhea Dan? It’s been up for debate the last few days since they were found. Who knows how long they’ve been there in the first place. Mostly likely, from the ground, you can’t tell they’re there. But this might be the most authoritative explanation.
According to Jonathon Hill, a Mars mission planner and technician, “The grids of zigzagging white lines seen in two of the images — the strangest of the various desert structures — are spy satellite calibration targets. Satellite cameras focus on the grids, which measure approximately 0.65 miles wide by 1.15 miles long, and use them to orient themselves in space.”
(Fox – China Marks)